Create a composite image of all the visible img tags in the DOM and save it to the app data directory of a Windows 8 Application

createAndSaveCompositeImage.js

    function SaveSplash() {

        window.msWriteProfilerMark("start-render");

        var body = document.body,
            appdata = Windows.Storage.ApplicationData.current,
            localfolder = appdata.localFolder,
            canvas = document.createElement("canvas"),
            ctx = canvas.getContext("2d"),
            imgs = document.body.getElementsByTagName("img"),
            size = imgs.length,
            i = 0, box, img, blob;

        canvas.width = body.clientWidth;
        canvas.height = body.clientHeight;

        for (var i = 0; i < size; i++) {
            img = imgs.item(i);

            try {
                box = img.getBoundingClientRect();
            } catch (e) {
            }

            if (!!box &&
                box.left <= canvas.width &&
                box.top <= canvas.height &&
                box.left + box.width >= 0 &&
                box.top + box.height >= 0) {
                ctx.drawImage(img, box.left, box.top, box.width, box.height);
            }
        }

        window.msWriteProfilerMark("start-encode");

        blob = canvas.msToBlob();
        img = blob.msDetachStream();

        window.msWriteProfilerMark("start-write");

        return localfolder.createFileAsync("splash.png", Windows.Storage.CreationCollisionOption.openIfExists).then(function (file) {
            return file.openAsync(Windows.Storage.FileAccessMode.readWrite);
        }).then(function (ras) {
            var os = ras.getOutputStreamAt(0),
                is = img.getInputStreamAt(0);

            return Windows.Storage.Streams.RandomAccessStream.copyAndCloseAsync(is, os);
        });
    }

// ...

    app.oncheckpoint = function (args) {

        window.msWriteProfilerMark("start-suspend");

        args.setPromise(SaveSplash().then(function () {
            window.msWriteProfilerMark("stop-suspend");
        }));
    };